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Conformation Changes and Luminescent Properties of Au-Ln (Ln = Nd, Eu, Er, Yb) Arrays with 5-Ethynyl-2,2′-Bipyridine

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journal contribution
posted on 17.11.2008, 00:00 by Hai-Bing Xu, Li-Yi Zhang, Jun Ni, Hsiu-Yi Chao, Zhong-Ning Chen
Reaction of polymeric gold(I) acetylide species (bpyC≡CAu)n (bpyC≡CH = 5-ethynyl-2,2′-bipyridine) with diphosphine ligands Ph2P(CH2)nPPh2 (n = 2−6) or 1,1′-bis(diphenylphosphino)-ferrocene (dppf) in dichloromethane induces isolation of binuclear gold(I) complexes (bpyC≡CAu)2{μ-Ph2P(CH2)nPPh2} or (bpyC≡CAu)2(μ-dppf). Complexation of Ln(hfac)3 (hfac = hexafluoroacetylacetonate, Ln = Nd, Eu, Er, Yb) subunits to the binuclear gold(I) complexes through 2,2′-bipyridyl chelation gives the corresponding Au4Ln4 or Au2Ln2 heteropolynuclear complexes. Noticeably, upon formation of the Au4Ln4 arrays by complexation of (bpyC≡CAu)2(μ-Ph2P(CH2)4PPh2) (3) with Ln(hfac)3 units, trans-conformation in 3 transforms dramatically to the cis-arranged form due to the strong driving force from ligand-unsupported Au−Au contacts between two Au2Ln2 subunits. In contrast, cis-conformation in (bpyC≡CAu)2(μ-dppf) (6) stabilized by Au−Au interactions is reversed to the trans-oriented form upon formation of Au2Ln2 arrays by introducing Ln(hfac)3 units through 2,2-bipyridyl chelation. The binuclear gold(I) complexes show bright blue luminescence featured by ligand-centered π → π* (C≡Cbpy) states together with low-energy emission at 500−540 nm, associated with 3(π→π*) excited states, mixed probably with some characteristic from (Au−Au) → (C≡Cbpy) 3MMLCT transition. For Au4Ln4 or Au2Ln2 complexes, sensitized lanthanide luminescence is achieved by energy transfer from Au-acetylide chromophores with lifetimes in the sub-millisecond range for EuIII complexes, whereas in the microsecond range for near-infrared emitting NdIII, ErIII, and YbIII species.

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